The Okhla slums of Delhi are home to 150,000 of India's poorest citizens – and malnutrition is a daily crisis. So how did a former Vogue cover star come to visit – and does the celebrity photo-op really have a lasting charitable effect?

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The common touch: Erin O'Connor talks to Shyama, 30, and her severely malnourished son Dilkhush, 15 months, at her home in the Tonk region in Rajasthan, India

SUSANNAH IRELAND

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Beena Misra with Mahima, her 15-month-old granddaughter, in the Okhla slums. Mahima, who has never had vegetables or milk, is the size of a six-month-old

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Naushad, aged five, is weighed at the Malnutrition Treatment Centre in the Kail Patan region of Tonk; he is 92cm tall and weighs just 8kg

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Digging in: O'Connor joins local children as they have their daily meal at the Save the Children education centre

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The Save the Children education centre is located in the Okhla slums

SUSANNAH IRELAND

The common touch: Erin O'Connor talks to Shyama, 30, and her severely malnourished son Dilkhush, 15 months, at her home in the Tonk region in Rajasthan, India

It's not often that a celebrity is embarrassed by the size of their entourage. Usually it's the bigger, the better – a signal of enduring popularity and profile. But today Erin O'Connor is visibly wincing, horrified by the pack of people who seem to have picked up her trail. As a newly recruited ambassador for Save the Children, the willowy British supermodel is visiting the sprawling Okhla slums of South Delhi, home to 150,000 of the subcontinent's poorest. Already she is having trouble inching her 6ft frame through its labyrinth of alleyways. And already she is keenly aware of the sensitivity of the situation. The last thing she needs is a gaggle of wide-eyed followers.

Okhla is a precarious sprawl of roughshod brickwork and corrugated roofing; how it all stays up is anyone's guess. Every square inch is a space eked out in which to live – neighbours are often separated from each other by nothing but a curtain. There is no running k water and no toilets. Instead, at its heart lies a stinking, twitching open sewer the size of a small river, along the edges of which children and rats play. Men defecate openly here, and women, who are not allowed that pleasure, just have to hold on until after dark.

O'Connor's discomfort becomes more acute as she is introduced to an elderly woman called Beena, who ushers her into her home and insists she stay for lunch. Beena's place is a 6ft square home to six other family members. On the menu, as it is most days, is a bowl of rice. O'Connor finally manages to ditch the hangers-on as she squeezes through the doorway and plonks herself down on the concrete floor, trying to find space in which to place her long, elegant limbs. She is given the task of stirring the rice in a pot that bubbles perilously close to the brim.

"I've never been so nervous in my life; I was convinced that I was going to spill the lot," she tells me later. "It's a very intimate thing to invite someone into your home; there's a lot of trust involved. That's why I wanted to be able to have time alone with her away from everyone else."

The Vogue cover star's visit comes as part of Save the Children's A Life Free From Hunger campaign; launched last week, it is one of the biggest actions in its near-100-year history. The aim is to highlight the world's "hidden hunger" – so it's not about the graphic images and swollen tummies seen when famine strikes, but rather the everyday, grinding hunger that is felt by millions of children worldwide.

In some ways this kind of hunger can be just as deadly, because it's assumed that if a child has a bowl of rice they're doing OK – but actually the damage done by malnutrition can last a lifetime. As well as causing stunted growth, the condition can lead to severe mental-health problems and also means easily treatable illnesses such as pneumonia and measles become killers. Although India is one of the world's biggest boom economies, nearly two million of its children die before the age of five – the highest rate in the world – about half of which are due to malnutrition.

Beena's family is a case in point. Her granddaughter Mahima is a tiny bow-legged little thing who is 15 months' old but still just the size of a six-month-old. She lies listless in her grandmother's arms and is highly susceptible to the multitude of illnesses that arises from living on the banks of a cesspit. Beena's family, like many in Okhla, are migrants who came pouring into the city from Uttar k Pradesh, Rajasthan, West Bengal and Bihar – one of the poorest states in India – in search of a better life. Beena says the family she left behind in her home village laugh at her because of the dire situation she has ended up in. Back in Bihar, she had a small plot of land on which to grow vegetables. Here, her diet consists almost solely of bread and wheat. And there is a strict pecking order in which her family is fed: her son gets the lion's share, then her daughter-in-law, followed by the two grandchildren and finally Beena herself, who gets to eat just once a day.

"Whatever I anticipated it would be like," says O'Connor, "this is 10 times worse. There is an unbelievable, unforgiving quality here; these people are trying to make the best of the most desperate conditions. It hit me with such impact."

The model is an inspired choice for Save the Children. She first got involved a little over a year ago when she donated a handbag to one of the charity's online auctions and found herself further drawn in. "As I began to get a deeper understanding of the vital life-saving work Save the Children does, I felt compelled to help in any way I could," she says. "This is about safe-keeping, inspiring and empowering a future generation – to facilitate them to make their own lives a little bit better. It was important for me to come here to see for myself, so important."

She has ploughed her way through the three huge files of information the charity gave her and is dedicated to making the most of her week here. At the end of a long, hot day trawling the streets, when others would be in the hotel bar, she finds time to squeeze in a last-minute interview with The Times of India – sample question: how do you think holding a malnourished baby is going to advance your career?

She is also a good choice, because she has the sense to step back and consider what a glamorous millionaire wandering among India's poor can look like. "I am aware it's easy and may be fashionable to pose with a slum child, and the irony of getting the media along means that it can come across as disingenuous," she says. "But you take these things on board and you hope you mean it whenever you get stuck into something. It's a big shame that when you have a platform to write about Save the Children, the media interest lies with my moral alignment."

O'Connor recently turned 34 and, aside from a small break a year-and-a-half ago – "I just wanted to have the luxury of sitting at home doing nothing" – has been modelling for 15 years, which makes her a virtual elder stateswoman of her profession. She is also one of a rare breed in the modelling world willing to speak her mind. Five years ago she set up the Model Sanctuary, a pioneering refuge for models, where therapists and nutritionists are among a team of practitioners who treat up to 300 girls a day during London Fashion Week. It's one of few visible signs of the industry addressing the welfare of its own, in a business notorious for keeping schtum on its darker side. She has spent a year raising sponsorship money for the organisation, but has just heard she has lost the building in which it is housed. "This is not just a fluffy little place where you can get a massage," she says. "It's about the welfare of human beings."

She also continues to be highly vocal about the size-zero debate. "I can't even get sample sizes over my knees," she says, as someone who clearly loves her food. Over breakfast she discusses the impact of the influx of younger, skinnier Eastern European girls on her industry and how certain photographers, who she says come on to their models as a matter of course, don't bother with her because they assume she's a lesbian. The industry is clearly a subject dear to her heart; it's little wonder Karl Lagerfeld describes her as one of the best models in the world.

On day two we head out to rural Kali Paltan, near Tonk, a town in Rajasthan, 60 miles south of Jaipur. Here, poverty is rife and the main source of income seems to be rolling cigarettes, where making 1,000 a day will earn just 50 rupees (65p). O'Connor, now getting into her stride, banishes everyone – half the village seem to be on her tail – and disappears into the house of Shyama, a 30-year-old mother of two. She stays for nearly an hour talking to Shyama about her husband, who works cleaning drains; her malnourished one-year-old who still hasn't learnt how to sit up; and her seven-year-old, who, in between working as a rag-picker (plastic-bag collector), takes sole responsibility for the seriously ill baby while his parents work. It's a bleak picture, made even more so by the fact that Shyama's family are Dalit, which in the Indian caste system is more commonly known as "untouchable".

We go on to the market and see the very real effects of globalisation. The economic downturn has had a ripple effect out here and so, too, has the global rise in food costs. In India, prices have gone up by a massive 17 per cent, and in rural parts, where four out of five people are living on less than a dollar a day, this is life-threatening. It has now reached such a crisis point that the Indian government is urgently debating a Right to Food bill, which would create a law guaranteeing every person in India access to minimum amounts of rice and grain.

"There was a time when you could get a whole range of vegetables for five to 10 rupees; now nothing is below 20 rupees [25p]. Milk, too, is now out of the bounds of ordinary people," says Anil Bhardwaj of the Centre for Community Economic Development Consultants Society. "It used to be said that the poor man in India could at least eat dahl and roti. Now they can't even afford that."

We go on to a malnutrition centre where a group of women sits on dirty beds clutching their children, while a cow wanders about in the corridor outside. O'Connor meets Harris. "[Harris] was the most extreme case of malnutrition I've seen," she says. "He was a four-year-old wearing the clothes of a six-month-old. His hunger had affected his mental health so even the tiniest gestures were hard to detect; everything he did looked like it was happening in slow motion. But Harris is just the tip of the iceberg – he's just one thing I've seen in five days. It's such a huge issue, it's quite overwhelming."

As the week goes on, O'Connor visibly grows in confidence. When someone suggests she ask the mother of a malnourished girl to compare her diet to that of a Western child, she refuses on grounds of tact. And upon hearing that a number of local men have got involved in a Save the Children centre in a predominantly Muslim community, she insists on meeting them. "I think it's brilliant they are involved and a really big deal," she says, "Decision-making within the family rests with the man [in these communities], so their involvement is crucial."

Striding down the back-streets of rural India, O'Connor cuts an incongruous sight. With her bobbed hair and androgynous air she is the antithesis of what is considered attractive here, and she laughs dismissively when someone worries that she may be getting hassled by the men. "I have been touched quite a lot on this trip," she says, "but it's because my skin is so white. Sometimes it's almost reflective against the sun."

She is quizzed about why she keeps her hair short and why, at the ripe old age of 34, she isn't yet married. A couple of women stop to bless her in the hope she has children. Although she is an aunt twice over and godmother to six, it is something, she tells me, her own grandmother is also slightly fixated on, too. "Granny thinks there is another side to me I should be working on," she laughs. "It would be fantastic to be a mum. I can't imagine going through life without children, but it hasn't happened yet because I haven't been ready."

O'Connor herself comes from a large Irish Catholic family (her name actually translates as "from Ireland"), but was raised in a small town just outside Birmingham. Her father was one of 13, two of whom died in infancy. "We lived in an Irish community where everybody knew everybody," she says. "Our version of normal was for everyone to be living on top of one another and for there to be lots of children flying around. It was a right laugh. We grew up in modest circumstances; my dad remembers moving into a two-up, two-down which actually had running water. Not for a moment am I suggesting that it's comparable [to the situation here] – I have been given the space and education for ambition – but I think it's important to have an awareness of where we begin."

O'Connor's visit to India is charity at work in the modern age. Where once we had begging boxes, we now have glamorous young celebrities using their profile to raise awareness. In this month's issue of Red magazine we can see actress Romola Garai in Burundi, while Myleene Klass is in Bangladesh in The Sun. Aside from the case of Big Brother contestant Darren Ramsay being sent by Christian Aid to Jamaica, where he promptly complained of feeling unwell and went home, it's done because it works. There is a direct correlation between the attention a celebrity trip will create and the spike in donations that follows. So when you see the Save the Children Mobile Health unit parked up in the Okhla slum, as it is once a week, and Erin O'Connor sat in the back with the forthright Dr Duggan, who is dishing out life-saving Vitamin A and iron tablets, this is where the whole enterprise has come full circle.

It's interesting to see what O'Connor will take away from the trip. She emails me afterwards to say she felt a tidal wave of emotion when she hit home turf and has been suffering, unusually for her, from bad jet lag. But, she says, there is no conclusion because, as far as she's concerned, her work in India has only just begun.

"Next week I'll be glammed up on set in New York," she says. "There really is no reconciliation between those two worlds to be made on any level whatsoever, other than it's me being looked at again. But fashion is probably the most visual industry in the world and imagery the most powerful tool. My plan is to know a bit more, be a bit more, try a bit harder. I have to compartmentalise and I have to be realistic, but the plan is to fully utilise the industry that I am in, in order to make an impact."